The present invention generally relates to determination of patient-specific ocular parameters. The availability of a complete patient-specific eye model would provide many advantages in vision science and ophthalmology. A patient-specific eye model could be used as a basis for vision correction of higher-order aberrations in laser refractive surgery or with corrective lenses, since classical devices only measure and correct for basic refractive errors (i.e., defocus and astigmatism). Accurate determination of optical parameters in normal and abnormal eyes would be useful in developing data bases for clinical diagnosis of pathologies. Measurements of ocular surface misalignments would be useful after implantation of intraocular lenses in cataract surgery. Moreover, knowledge of the refractive index distribution in the lens would be beneficial in optical coherence tomography, which is based on interferometric reflectometry and index changes. It could also lead to a substantial improvement in retinal imaging.
In addition to improvements in vision correction and retinal imaging, the availability of patient-specific parameters could facilitate a broad range of ongoing vision science studies. Of interest is the in vivo gradient-index (GRIN) distribution and lenticular geometry of the human crystalline lens as a function of both age and accommodation, but this information has been difficult to obtain, and reliable measurements are scarce. While previous studies suggested that aspheric surfaces in the anterior segment and an effective refractive index for the lens are sufficient to model spherical aberration, lack of knowledge regarding the GRIN distribution precludes both the prediction of off-axis aberrations and study of dispersion in the lens, so experimental data are limited. A complete mapping of the human eye could also be used to evaluate intersubject variability and statistical variations, as well as vision performance and image quality in the central and peripheral visual fields, which could be enhanced by accurate measurement of the retinal curvature. Another fundamental study in physiological optics is how individual ocular components factor into the overall performance of the human eye and how such performance would change if one or more surfaces are altered, a critical element in surgical procedures.
Current theoretical eye models often lack contributions from asymmetry such as decentration of the lens or pupil, which manifest in the fovea as aberrations of non-axisymmetrical systems (e.g., coma, astigmatism, and transverse chromatic aberration) and may have a significant impact on ocular performance.
Several methods to determine aberrations of the eye are reported using wavefront data. These methods generally analyze wavefront data generated using a wavefront sensor, such as a Shack-Hartmann wavefront sensor, and model the wavefront using Zernike polynomials, for example, through known fitting techniques. The shape of the wavefront can be used to determine ocular aberrations. A technique has been described using wavefront data obtained at multiple excitation angles of the eye to generate information about the sources of aberration in the eye using mathematical modeling. (Goncharov, et al., Laser Physics, 2006, Vol. 16, No. 12, 1689-1695). U.S. Pat. No. 6,525,883 describes an apparatus used to measure one optical characteristic such as refracting power or shape of the cornea by illuminating a small area of the retina and calculation of Zernike factors from a split reflected light beam. U.S. Pat. No. 7,270,413 describes a method to calculate an optical characteristic of the eye by illuminating a portion of the retina, converting the reflected light into at least 17 beams and calculating Zernike coefficients using measured values of refractive power distribution and pupil diameters in the calculation. U.S. Pat. No. 6,802,609 describes methods of measuring a characteristic of the eye by illuminating a small region of the retina and calculating Zernike coefficients from the reflected data using a reference optical path in the system to account for equipment error. U.S. Pat. No. 6,273,566 describes a method for measuring a characteristic of the eye by calculating how an image is observed by the eye. U.S. Pat. No. 7,311,402 describes measuring the amount of scattering from the crystalline lens and retina in a wavefront sensing system. US 2007/0216866 describes using polarized light to calculate wavefront aberrations of an eye. US 2004/0257530 and US 2007/0222948 describe using Fourier transformation of wavefront data from the eye to calculate an optical surface model for use in laser eye surgery. U.S. Pat. No. 7,034,949 describes a method for determining the shape of a wavefront from an object by fitting Zernike polynomials to a diffraction pattern produced by the object. US 2004/0263779 describes a wavefront sensing system which uses a CMOS imaging system. U.S. Pat. No. 7,113,268 describes a wavefront sensing system which addresses spatio-temporal fluctuations (scintillation) in the intensity of the wavefront by using two intensity measurements. U.S. Pat. No. 5,384,455 describes methods for obtaining fine-resolution images in the presence of time-varying aberrations by collecting multiple images of the object and aberrations and estimating the object image. A mathematical wavefront aberration function was described using measured physical properties of anterior chamber length, axial length and cornea topography, and modeling light distribution in the eye (Rouarch, et al.,Journal of Modern Optics, 2008, Vol. 55, No. 4-5, 717-725). By illuminating the eye at different angles from the optical axis, measuring the wavefront aberrations of each beam and reverse ray tracing, a model eye was calculated (Goncharov, et al., Optics Express, 2008, Vol. 16, No. 3, 1692.
Techniques developed for use in astronomy have been adapted to ophthalmology. For example, modal tomography is used in astronomy to calculate a three-dimensional distribution of aberrations caused by turbulence using laser guide stars. A SLODAR (Slope Detection and Ranging) technique has been described which generates point source “stars” by using a reference laser beam to scatter from the retina and mathematical modeling the resultant waveforms (Lambert et al., Optics Express, 2008, Vol. 16, No. 10, 7309). A “prescription retrieval” system was used to estimate the optical parameters of optics on the Hubble Space Telescope as compared to the intended design parameters by varying parameters in an initial mathematical model of the optical component until images generated by the model match the actual images obtained from the actual optics system (Redding, et al., Applied Optics, 1993, Vol. 32, No. 10, 1728).
Current adaptive-optics ophthalmoscopes incorporating a Shack-Hartmann wavefront sensor (WFS) and wavefront corrector conjugated to a single surface of the eye offer high resolution, but over a very limited field-of-view (FOV) due to a form of anisoplanatism involving aberrations of the eye. Aberrations collected over different field positions on the retina result from the passage through different parts of the ocular media so that the adaptive-optics correction is valid only over a certain field area, referred to as the isoplanatic patch. One solution is to conjugate multiple wavefront sensors and correctors to various refractive surfaces in the eye, thereby increasing the isoplanatic patch size and enabling wide-field measurements, but choice of the optimal planes at which to conjugate the correctors would be facilitated by knowing the real eye structure of the individual. Mathematical averaging and physical methods (immersion of the eye) to enlarge the isoplanatic patch size have been proposed (Dubinin, et al., Journal of Modern Optics, 2008, Vol. 55, No. 4-5, 671-681).